(a) Field of the Invention
The present invention relates to zip fasteners and more particularly to a double-layer nylon zipper, which is practical for use in a travel bag, backpack or tent.
(b) Description of the Prior Art
Most conventional nylon zippers are of a single layer design. A single-layer nylon zipper is not strong enough. Simply inserting the tip of a ball pen or a sharp end of a rod into a gap between the two interlocked series of teeth of a single-layer nylon zipper can open the zipper easily. Thus, using a single-layer nylon zipper in a travel bag or the like cannot effectively protect stored items against loss or theft. To solve this problem, double-layer nylon zippers are developed. For example, Taiwan Patent Publication Number 441257 discloses a double-layer nylon zipper, which has an interlocking series of teeth arranged at each of the opposing top and bottom sides of each zipper tape to provide an anti-piercing function.
However, in conventional double-layer zipper designs (such as U.S. Pat. No. 8,132,303, U.S. Pat. No. 7,891,061), the opposing top and bottom walls of the zipper slide have the same length. When a double-layer zipper is used in a corner of a travel bag or backpack, the difference in close angle between the two layers of interlocking teeth causes the double-layer zipper unable to overcome the strong tension during closing of the two layers of interlocking teeth in the corner area, leading to one layer of the interlocking teeth closed and the other layer of interlocking teeth burst open.
Further, the interlocking series of teeth of each zipper tape of a double-layer nylon zipper is formed of a left-handed or right-handed series of coil elements of a plastic wire material and fastened to the narrow elongated fabric strip of the respective zipper tape with a respective stitching line. When moving the zipper slider relative to the zipper tapes, the stitching lines of the zipper tapes are kept in direct contact with the zipper slider. Friction between the top and bottom stitching lines of the zipper tapes and the zipper slider can damage the stitching lines, shortening the lifetime of the zipper. Therefore, conventional zippers have numerous drawbacks.